Electric heating resistors



1967 N76. SCHREWELIUS ET AL 3,345,597

ELECTRIC HEATING RESISTORS Filed Aug. 24, 1964 Fig.1

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INVENTOR ATTORNEY United States' Patent 3,345,597 ELECTRIC HEATING RESISTORS Nils Gustav Schrewelius and Clas Bengt 0. Magnusson, Hallstahammar, Sweden, assignors to Aktiebolaget ganhal, Hallstahammar, Sweden, a corporation of we en Filed Aug. 24, 1964, Ser. No. 391,555 priority, application Sweden, Aug. 27, 1963,

9,321/63 7 Claims. (Cl. 338-302) The present invention relates to electric heating resistors manufactured in a powder-metallurgical way and composed of a conductive tubular resistor element and a support therefor. Said element consists substantially of molybdenum silicide, particularly MoSi and, if desired, also of one or more other heat resisting silicides and/ or oxides, such as SiO and is slotted along a helix to ob tain a helical resistor portion and thus increase the electrical resistivity thereof so as to make said portion to constitute an incandescent zone. Said support is constituted by a heat resisting through-going elongated body or rod inserted into said element. Those portions of the resistor element which are not cut up have a lower resistivity and serve as relatively cold terminals.

Suitable materials for the conductive resistor element are disclosed in the United States Patents Nos. 2,992,959, 2,993,111, 3,027,330, 3,027,331, 3,027,332 and 3,036,017.

Suitable materials for the support are described, for instance, in the French Patent No. 1,320,277. Such materials should have a sufficiently high content of silicon carbide to guarantee an adequate stiffness of the support at the operating temperature.

In the resistor element there may also be included a certain content of SiC which should be so low or at most 20% by volume in order to make sure that the electric conductivity at room temperature is not reduced too much. Such a material becomes soft at high temperature, such as up to 1600 C., and a resistor element made of this material can thus not be mounted in a horisontal position. It has been proposed to support such a tubular resistance element upon a through-going interior silicon carbide rod which has a silicon carbide content exceeding about 40% by volume. The carbide particles are recrystallized and constitute a skeleton the cavities of which are impregnated with a silicide. Such a rod will retain its stiffness also at very high temperatures. On the inside of the resistor element and on the outside of the support there will then be formed a quartz glass film which at high temperature cements together the resistor element and the support. As the coeflicient of thermal expansion is quite different in the resistor element and in the support this will produce mechanical tensions between the resistor element and the support body resulting in ruptures in the material when the temperature changes. 7

The present invention has, inter alia, for its purpose to avoid this inconvenience. This is obtained by interposing an intermediate layer between the resistor element and the support, said layer being electrically insulating and forming a non-adhesive intermediary between the surfaces.

Surprisingly, it has been found that the quartz glass film does not adhere to said intermediate layer it this is made of mullite. In this case the resistor element and the support obtain a certain degree of freedom of movement in relation to one another, and are thus relieved Claims from thermal tensions. This freedom of movement is of particular importance in this case in which the central zone is weakened by the helical slot.

On the accompanying drawing there is shown an embodiment of the invention, FIG. 1 being a lateral view and FIG. 2 an end view.

The incandescent zone of the resistor element 1 is constituted by the central portion thereof and is provided with a helical slot 2 so that the electrical resistivity in this zone is several times higher than in the terminal zones. Along the central axis of the element the support extends in the shape of a rod 3 and between this rod and the element there is an intermediate layer 4 interposed. This intermediate layer may consist of material which is flame sprayed directly on the support rod. It may also be constituted by one or more tube sections thrust on the support rod 3.

What we claim is:

1. A rod-shaped electrical heating resistor composed of a generally cylindrical resistor element consisting essentially of molybdenum disilicide, said cylindrical resistor element containing a helical slot in the central portion thereof to form a helical resistor element and thereby increase said elements electrical resistance in said central portion, said helical resistor element being selfsupporting at normal room temperature and being deformable by the influence of its own weight when unsupported during operation at high temperatures between 1400 C. and 1800" C.; a rod-shaped heat-resisting support inserted into said cylindrical resistor element to prevent said elements high temperature deformation, and an intermediate layer of insulating material positioned between the supporting rod and the resistor element, the outside surface of said intermediate layer being in contact with the inner surface of said cylindrical resistor element.

2. An electrical heating resistor as described in claim 1, wherein the outer surface of said intermediate material is in non-adhesive cont act with the inner surface of the cylindrical resistor element.

3. The resistor of claim 1 in which said intermediate layer is made of mullite.

4. A resistor as claimed in claim 1 in which the intermediate layer is made in the form of one or more tubes thrust on to the support body.

5. Resistor according to claim 1 in which the intermediate layer consists of a film flame sprayed directly on to the support.

6. A rod-shaped electrical heating resistor composed of a generally cylindrical resistor element consisting essentially of molybdenum disilicide, said cylindrical resistor element containing a helical slot in the central portion thereof to form a helical resistor element and thereby increase said elements electrical resistance in said central portion, said helical resistor element being selfsupporting at normal room temperature and being deformable by the influence of its own weight when unsupported during operation at high temperatures between 1400 C. and 1800 C.; a rod-shaped heat-resisting support inserted into said cylindrical resistor element, said rod-shaped support being composed of insulative material.

7. An electrical heating resistor as described in claim 6, wherein the outer surface of the support rod is in nonadhesive contact wit h the ir lner surface of the cylindrical 7 2,622,304 12/1952 Coffer 252--518 resistor element.-- 1 1 i 3,063,100 11/1962 K-Ohring 338226 X References Cited 3,254,320 5/ 1966 Hill et a1. 338-331 UNITED STATES PATENTS 5 RICHARD M. WOOD, Primary Examiner. 1,551,766 9/1925 North-Ind 1325 X 2,298,791 '7/1942 Loftis et a1. 338-314 X SMITH Assistant Examiner- 

1. A ROD-SHAPED ELECTRICAL HEATING RESISTOR COMPOSED OF A GENERALLY CYLINDRICAL RESISTOR ELEMENT CONSISTING ESSENTIALLY OF MOLYBDENUM DISILICIDE, SAID CYLINDRICAL RESISTOR ELEMENT CONTAINING A HELICAL SLOT IN THE CENTRAL PORTION THEREOF TO FORM A HELICAL RESISTOR ELEMENT AND THEREBY INCREASE SAID ELEMENT''S ELECTRICAL RESISTANCE IN SAID CENTRAL PORTION, SAID HELICAL RESISTOR ELEMENT BEING SELFSUPPORTING AT NORMAL ROOM TEMPERATURE AND BEING DEFORMABLE BY THE INFLUENCE OF ITS OWN WEIGHT WHEN UNSUPPORTED DURING OPERATION AT HIGH TEMPERATURES BETWEEN 1400*C. AND 1800*C.; A ROD-SHAPED HEAT-RESIST- 